The invention relates to an apparatus for detecting ventricular tachycardia. Such an apparatus can be made in the form of an independent diagnostic unit or as a component of a cardiac pacemaker with an antitachycardia operating mode or in the form of a defibrillator or cardioverter.
The treatment of tachycardial disturbances in heart rhythm is one of the most important tasks in cardiology and in particular is a primary field for the use of electrical stimulation of the heart. In view of the manifold forms such problems can take and their manifold causes, extensive attempts have been made for years to improve the preconditions for therapeutic success by refining the classification of tachycardial arrhythmias and associating them more exactly with typical defects in the cardiac stimulus conduction system.
Because pathological ventricular tachycardias (hereinafter also abbreviated "VT") are so widespread and so needful of treatment on the one hand, and because they have a good chance of successful treatment with special pacemaker pulse trains on the other, reliably distinguishing them from other arrhythmias, and especially physiological tachycardias, ventricular fibrillations ("VF") or supraventricular tachycardia ("SVT")--including under special circumstances--has proven to be an especially important problem.
In German Patent Application DE-A 44 39 256, it is proposed that the relative distribution of heartbeat intervals, which can be considered as fibrillation intervals or tachycardia intervals, within predetermined time ranges can be utilized to classify the prevailing type of arrhythmia. This is intended to take proper account of the fact that in practice, the interval lengths in ventricular tachycardia on the one hand and fibrillation on the other often overlap considerably.
A great many other approaches--which will not be described in detail here--to solving these problems seek to employ analysis of the three-dimensional propagation or correlation of depolarizations in the cardiac tissue. This requires implanting many electrodes for signal detection--sometimes even pericardially--and for this reason, if for no other, has little chance of being realized.
In U.S. Pat. No. 4,860,749, a method for distinguishing ventricular tachycardia from a sinus or other kind of supraventricular tachycardia is described, in which the atrial heart rate and the ventricular heart rate (the inverse values of each will hereinafter also be called the "P--P interval" or "R--R interval") as well as the A-V interval (hereinafter sometimes also called the "P-R interval") are measured. If the R--R interval is within a predetermined range and is shorter than the P--P interval, then the condition is readily classified as ventricular tachycardia. If the atrial and the ventricular rate are approximately the same as a consequence of 1:1 A-V conduction or retrograde conduction, then the measured A-V interval is subjected to a comparison with a predetermined value ("sinus A-V interval"), and the classification criterion is obtained from the outcome of the comparison.
In U.S. Pat. No. 5,325,856, a method for distinguishing between ventricular and supraventricular tachycardias is proposed, which is based on a comparison of the divergence over time in the P-R and R--R values with two predetermined threshold values at the onset of the tachycardia.
U.S. Pat. No. 5,327,900 describes a method for distinguishing between pathological and physiological (sinus) tachycardias at a comparable atrial and ventricular rate, which is based on the association of the measured A-V interval with a predetermined A-V time slot, which has been determined from the A-V interval during normal sinus rhythm. This algorithm is comparatively simple; however, thus far there is no proof of its being efficient enough to make such distinctions.
European Patent Application EP-A 0 597 459 describes a method in which if the R--R and P--P intervals agree, a comparison is first made between the length of the A-V (P-R) interval and a predetermined base value, and finally--if that comparison does not produce a conclusive result--a test stimulation in the atrium is performed, and the classification is made on the basis of the stimulated heart response, in particular changes over time in the R--R intervals.